As advances have been made in micro-machining techniques, the need for microactuators has increased in various fields. Optical switches which switch optical paths utilized in optical communications, etc., may be cited as one example of a field in which microactuators are used. For instance, the optical switch disclosed in Japanese Patent Application Kokai No. 2001-42233 may be cited as one example of such an optical switch.
Microactuators generally have a fixed part, and a movable part that can be moved by a specified force, and are held in a specified position by the specified force. In conventional microactuators, an electrostatic force is often used as the specified force. For example, in the case of the microactuator that moves a micro-mirror used in the optical switch disclosed in Japanese Patent Application Kokai No. 2001-42233, the movable part can be moved to an upper position (position in which the micro-mirror reflects the incident light) or a lower position (position in which the micro-mirror allows the incident light to pass through “as is”), and can be held in these positions, by an electrostatic force.
In such microactuators that utilize an electrostatic force, a first electrode part is disposed on the fixed part, a second electrode part is disposed on the movable part, and an electrostatic force is generated between the first and second electrode parts by applying a voltage across these electrode parts.
In the case of conventional microactuators using an electrostatic force as described above, the movable part is moved by an electrostatic force and held in a specified position by an electrostatic force; accordingly, it is difficult to broaden the range of mobility of the movable part.
The electrostatic force F1 that acts between parallel flat-plate electrodes is as shown in Equation (1) below, where ε is the dielectric constant, V is the potential difference, d is the inter-electrode distance, and S is the electrode surface area.F1=ε×V2×S/2d2  (1)
As is seen from Equation (1), the electrostatic force F1 decreases abruptly in inverse proportion to the square of the inter-electrode distance d as the inter-electrode distance d increases. Accordingly, in the case of the conventional microactuators, it becomes difficult to move the movable part when the inter-electrode distance d exceeds a certain distance, so that it is difficult to broaden the mobility range of the movable part. Furthermore, if the potential difference (voltage across the electrodes) V is increased in an attempt to obtain a sufficient electrostatic force F1 in the case of a large inter-electrode distance d, problems occur in terms of the dielectric strength, and a high-voltage generating part is required. Furthermore, if the electrode surface area S is increased in an attempt to obtain a sufficient electrostatic force F1 in the case of a large inter-electrode distance d, the dimensions of the device are increased, so that miniaturization, which is the whole idea of a microactuator, is lost.
According, as a result of research, the present inventors conceived of the use of Lorentz force instead of electrostatic force in a microactuator.
It is known that the Lorentz force F2 (N) is as shown in Equation (2) below, where B is the magnetic flux density (T), L is the length of the electric wire (m), and I is the current (A).F2=I×B×L  (2)
Since there is no term that stipulates the position of the electric wire in Equation (2), the Lorentz force F2 that is generated at a constant magnetic flux density does not vary even if the position of the electric wire changes.
The Lorentz force can be caused to act on the movable part in a microactuator by installing a current path corresponding to the electric wire in the movable part, applying a magnetic field to this current path, and causing a current to flow through this current path. Even if the mobility range of the movable part is broadened compared to that of a conventional device, the application of a substantially uniform magnetic field in this range can easily be accomplished, for example, by using a magnet. Accordingly, even if the mobility range of the movable part is broadened, a constant force can be caused to act on the movable part regardless of the position of the movable part. Specifically, if such a Lorentz force is used instead of an electrostatic force in a microactuator, a constant driving force can be obtained (in principle) regardless of the position of the movable part (unlike a case in which an electrostatic force which shows a variation in the driving force according to the position of the movable part is used).
For example, in the case of an inter-electrode distance of 50 μm, an electrode shape of 50 μm square, a voltage of 5 V, and a dielectric constant of 1, the electrostatic force F1 according to Equation (1) is 0.1 nN. On the other hand, if a current path with a length of 50 μm is created in a 50-μm-square electrode, and a magnetic field with a magnetic flux density of 0.1 T is applied, a Lorentz force of 5 nN is generated when a current of 1 mA is caused to flow. In order to obtain a force of 5 nN or greater using an electrostatic force, the inter-electrode distance must be set at 7 μm or less, or else the electrode shape must be set at 350 μm or greater. Accordingly, it is seen that the Lorentz force is more advantageous for obtaining the same driving force.
Furthermore, for example, if a 20-mm-square neodymium-iron-boron-type magnet is disposed in a position that is separated from the microactuator by a distance of 2 mm, a magnetic flux density of 0.1 T can easily be obtained.
Thus, the use of a Lorentz force instead of an electrostatic force in a microactuator makes it possible to expand the mobility range of the movable part without applying a high voltage or sacrificing compact size.
However, it has been demonstrated that a new problem arises in cases where a Lorentz force is used instead of an electrostatic force in a microactuator. Specifically, in cases where a Lorentz force is used instead of an electrostatic force, the movable part is moved to a specified position by means of this Lorentz force, and the movable part continues to be held in this position by the Lorentz force. Accordingly, since the current used to generate the Lorentz force must be constantly caused to flow in a continuous manner, the power consumption is conspicuously increased.
For instance, in the case of an application involving a large-scale optical switch, several tens of thousands of actuators are installed in a single optical switch device. Accordingly, there is a strong demand for a reduction in the power consumption of the respective actuators. For example, in the case of an optical switch with 100×100 channels, it is essential that (for example) MOS switches for selecting the channels be manufactured on a semiconductor substrate. Assuming that the resistance of one MOS switch is 10 kΩ, then in a case where a current of 1 mA is caused to flow continuously through this switch, the power consumption of one MOS switch is 10 mW. In a case where the total number of MOS switches is 10,000, the power consumption is as high as 100 W. As a result, the amount of heat generated is excessively large, so that there are problems in terms of practical use.
Furthermore, if it is possible to reduce the mechanical stress that is applied to the microactuator and a driven body by the shock or the like that accompanies the operation of the microactuator, then the useful life of this microactuator is extended, so that it is possible to increase the reliability during long-term operation, which is desirable. Moreover, it is desirable to increase the operating speed of the micro actuator.